This invention relates to a starter motor mainly used as a vehicular engine starter motor.
FIG. 2 is a partial sectional view illustrating a conventional stater motor disclosed in Japanese Patent Laid-Open No. 63-90665 for example. This starter motor is a coaxial starter motor in which an armature rotary shaft 2 of a d.c. motor 1, an output rotary shaft 4 having a pinion 3 at its front end portion (right end portion in the figure) and a solenoid switch unit (not shown) are arranged on the same axis. That is, the arrangement is such that the armature rotary shaft 2 is hollow, the plunger rod 5 of the solenoid switch unit disposed behind the d.c. motor 1 is inserted into an inner passage 2a of the armature rotary shaft 2, and the output rotary shaft 4 is disposed at the front end side of the inner passage 2a and the plunger rod 5 is brought into abutment with the rear end face through a steel ball 6, so that the output rotary shaft 4 can be pushed forward by the forward movement of the plunger rod 5.
Also, at the front end of the armature rotary shaft 2, a sun gear 7a of a planetary speed reduction gear 7 is formed. The planetary speed reduction gear 7 comprises the sun gear 7a, an inner gear 7b formed in an inner circumferential surface of a front bracket 8 and planetary gears 7d rotatably supported by pivot shafts 7c and meshing with the sun gear 7a and the inner gear 7b.
The pivot shafts 7c of the planetary speed reduction gear 7 are secured to a clutch outer member 9a of an over-running clutch mechanism 9 so that the speed-reduced output from the armature rotary shaft 2 is transmitted to the over-running clutch mechanism 9. On the inner circumference side of the clutch outer member 9a, a clutch inner member 9b and rollers 9c inserted between the clutch inner and outer members are provided, constituting the over-running clutch mechanism 9. In the inner circumferential surface of the clutch inner member 9b, helical spline grooves 9d are formed which are in mesh with the helical splines 4a formed in an enlarged diameter portion of the output rotary shaft 4, and a return spring 10 is disposed between a step portion 9e on the front end of the clutch inner member and the helical splines 4a for rearwardly biasing the output rotary shaft 4. Further, the front end portion of the clutch inner member 9b is supported by a bearing 11 fitted within the front bracket 8.
The pinion 3 is spline-engaged with straight splines formed in the front end portion of the output rotary shaft 4 and its forward movement is limited by a stopper 12. Also, within a recessed portion 3a formed in the inner circumference portion of the pinion 3, a pinion spring 13 is disposed between it and a step portion 4b on the output rotary shaft 4 so that the pinion 3 is urged forward. The pinion spring 13 is provided, after the stopper 12 is assembled for, always forwardly urging the pinion 3 and moderating the shocks upon contacting of the pinion 3 against the engine ring gear. Incidentally, 14 indicates a bearing disposed within the inner passage 2a of the armature rotary shaft 2 for supporting the rear portion of the output rotary shaft 4.
In the coaxial starter motor constructed as above-described, the rotational drive force of the d.c. motor 1 is transmitted to the over-running clutch mechanism 9 through the planetary speed reduction gear 7 and further to the output rotary shaft 4 which is spline-engaged with the clutch inner member 9b. As the plunger rod 5 is driven forward, the output rotary shaft 4 is moved forward so that the pinion 3 engages the unillustrated engine ring gear to start the engine. After starting, the operator manually turns off the solenoid switch to retract the plunger rod 5 to cause the output rotary shaft 4 to be returned to the original position (inactuated position) by the action of the return spring 10, thereby to disengage the pinion 3 from the engine ring gear. Also, the reverse driving from the engine side immediately after the starting of the engine is prevented from being transmitted to the side of the d.c. motor 1 by the unidirectional-clutch action of the over-running clutch mechanism 9.
The conventional starter motor is constructed as above-described and the spring 13 for forwardly biasing the pinion 3 is disposed within the recessed portion 3a in the inner circumference of the pinion 3 and on the outer circumference of the output rotary shaft 4.
Generally, since the deddendum thickness t of the pinion 3 and the effective diameter of the output rotary shaft 4 are difficult to make less than a predetermined value because the they must have a predetermined strength, the minimum deddendum diameter of the pinion 3 is restricted, so that the minimum number of teeth of the pinion 3 is also determined. More specifically, in a gear configuration of the level of DP10 (Module M=2.54), for example, which is widely used in the automotive engine ring gear and the pinion 3, the minimum number of teeth of the gear was conventionally eight.
On the other hand, in the starter motor of this type, a relationship expressed by the following equation is established between the volume of the armature of the d.c. motor 1 and the gear ratio between the pinion 3 and the ring gear: EQU Da.sup.2 Lc.varies.Te/g.multidot.I.multidot..sqroot.
where, Da: outer diameter of the armature core of the d.c. electric motor; Lc: armature core length (axial length of the core); Te: engine torque; g: gear ratio; I: drive current; Rs: resistors of the starter motor. As apparent from this equation also, the volume of the armature is in inverse proportion to the gear ratio between the pinion 3 and the ring gear, so that, in a situation in which the number of teeth of the ring gear is constant and the number of teeth of the pinion 3 is difficult to decrease, a reduction in volume of the armature, i.e., miniaturization and compactness of the starter motor, has been very difficult.
Also, in the conventional starter motor, the front portion of the output rotary shaft 4 is supported by a spline-engagement portion between the helical spline grooves 9d of the clutch inner member 9b and the helical spline portion 4a. However, since with the helical spline engagement it is difficult to make the clearance of the engaging portion extremely small because of the requisite slidable movement therebetween, a certain play is provided between the output rotary shaft 4 and the clutch inner member 9b, and since the engagement portion which is a support portion is not the front end portion of the clutch inner member 8b, the distance between the engagement portion and the pinion 3 is large, resulting in a large bending moment. Accordingly, since the conventional starter motor has the above-mentioned play and the large bending moment, it has the problems that a noise is generated during operation and the output shaft 4 may be broken. Moreover, since the helical spline grooves 9d provide sliding surfaces for the output rotary sahft 4 while supporting a load, this can cause undesirably poor sliding movement of the output rotary shaft 4 if the clearance is not properly selected.